1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting periodic channel state information in a wireless communication system.
2. Related Art
In order to maximize efficiency of limited radio resources, an effective transmission and reception scheme and methods of utilization thereof have been proposed in a wideband wireless communication system. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading. As a result, complexity is decreased in a receiving end and an interval of a transmitted symbol is increased, thereby minimizing the ISI.
In a system using the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing a part of available subcarrier to each user. In the OFDMA, frequency resources (i.e., subcarriers) are provided to respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the multiple users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for the multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
A multiple-input multiple-output (MIMO) technique uses multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Exemplary methods for implementing diversity in a MIMO system include space frequency block code (SFBC), space time block code (STBC), cyclic delay diversity (CDD), frequency switched transmit diversity (FSTD), time switched transmit diversity (TSTD), precoding vector switching (PVS), spatial multiplexing (SM), etc. A MIMO channel matrix depending on the number of receive antennas and the number of transmit antennas can be decomposed into a plurality of independent channels. Each independent channel is referred to as a layer or a stream. The number of layers is referred to as a rank.
Channel state information (CSI) may be transmitted through an uplink control channel (e.g., physical uplink control channel (PUCCH)). The CSI may include various types of information such as a channel quality indicator (CQI), a precoding matrix indicator (PMI), a rank indicator (RI), etc. The PUCCH carries various types of control information according to a format.
A carrier aggregation system has recently drawn attention. The carrier aggregation system implies a system that configures a wideband by aggregating one or more carriers having a bandwidth smaller than that of a target wideband when the wireless communication system intends to support the wideband.
There is a need for a method of transmitting various types of periodic channel state information, especially, multiple channel state information for a plurality of cells, in a carrier aggregation system. In particular, according to the conventional technique, if it is configured that different types of periodic CSI are transmitted in the same subframe, only a CSI having a high priority is transmitted or only a CSI for one of a plurality of cells is transmitted. Therefore, a periodic CSI feedback has a problem in its completeness.